In conventional video cameras, a system for recording a standard television signal obtained by photoelectrically converting an object image by an image sensing element has prevailed. It is a common practice to execute photoelectric conversion for a time corresponding to one period (field period) of a vertical synchronization signal of the standard television signal. However, a sufficient exposure time cannot often be assured within a predetermined field period, and a frame may suffer underexposure. To prevent such problem, especially in a low illuminance environment, a technique called slow shutter is put into practical use. With this technique, the exposure time of the image sensing element can be longer than one field period by devising the photoelectric conversion timing, and using an image memory.
The arrangement of an actual video camera will be described in detail below with reference to FIG. 1. Reference numeral 101 denotes a stationary first group lens; 102, a zoom lens that attains zooming; 103, an aperture; 104, a stationary second group lens; and 105, a focus compensation lens (to be referred to as a focus lens hereinafter) which has both a function of correcting movement of a focal plane upon zooming, and a focus adjustment function. Reference numeral 106 denotes a focus compensation lens motor as an actuator for moving the focus lens 105; and 107, a focus compensation lens driver for driving the focus compensation lens motor 106 in accordance with a signal from a camera control microcomputer 114 to be described later. Reference numeral 108 denotes a CCD serving as an image sensing element; and 109, a CCD drive circuit for driving the CCD 108. Reference numeral 110 denotes a camera signal processing circuit which processes an output signal from the CCD 108 to obtain a signal that is compatible to a recording device 112 to be described later.
Reference numeral 111 denotes an image memory, which stores a video signal from the camera signal processing circuit 110 as needed, and outputs the stored video signal to the recording device 112 to be described below. Reference numeral 112 denotes the recording device, which records an output signal from the image memory 111 for one field period. Nowadays, a magnetic tape is normally used as a recording medium. Reference numeral 113 denotes a display device which displays an output from the image memory 111. Reference numeral 114 denotes the camera control microcomputer, which controls the CCD drive circuit 109 and camera signal processing circuit 110, and sends a signal for driving the focus lens 105 to the driver 107 in accordance with an input from a manual focus dial 115 to be described below. Reference numeral 115 denotes the manual focus dial which converts rotation of a member into an electrical signal, and inputs the electrical signal to the camera control microcomputer 114.
In the video camera system with the arrangement shown in FIG. 1, the camera control microcomputer 114 generates a video signal by controlling the CCD drive circuit 109 and camera signal processing circuit 110, and sends a signal for driving the focus lens 105 to the focus compensation lens driver 107 in accordance with the input from the manual focus dial 115.
The slow shutter control of the video camera system will be explained below. The CCD drive circuit 109 normally drives the CCD 108 to make photoelectric conversion for one field period. However, in a low-illuminance environment, the CCD drive circuit 109 drives the CCD 108 to make photoelectric conversion after exposure across a plurality of fields, the output from the CCD 108 is processed by the camera signal processing circuit 110, the video signal is stored in the image memory 111, and the video signal is read out from the image memory 111 for one field period and is output to the recording device 112, as shown in FIG. 2. In this way, an image changes every plurality of field periods, but a bright image can be obtained. Note that the bracketed numbers in FIG. 2 indicate respective images.
Lastly, the manual focus control of the video camera system will be explained below. The camera control microcomputer 114 reads an electrical signal obtained by converting rotation of the manual focus dial 115, and sends a signal for driving the focus lens 105 in proportion to a change in electrical signal corresponding to the rotation amount of the manual focus dial 115 to the focus compensation lens driver 107, thus attaining the manual focus control. A photographer operates the manual focus dial 115 while observing the display contents on the display device 113, so as to attain manual focus operation.
FIG. 5 shows the control of the camera control microcomputer 114 at that time. This process is controlled to start from step S501 and to return to step S501 within one field. In step S501, the rotation direction and angle of the manual focus dial 115 in one field are detected. In step S502, the drive amount of the focus lens 105 for one field is determined in accordance with the rotation angle of the manual focus dial 115. In step S503, the focus lens 105 is driven by the drive amount determined in step S502 in a direction corresponding to the rotation direction detected in step S501. In this manner, the manual focus control is attained.
However, the conventional system suffers the following problem. Since the update period of a video signal is prolonged and it takes a long time until the photographer observes a video corresponding to his or her manual focus operation in the slow shutter mode, the focus lens has already overshot an in-focus position when an image in an in-focus state is output. For this reason, even when the photographer realizes such state and makes manual focus operation in the reverse direction, the focus lens repetitively overshoots an in-focus position, and the focal point position cannot be determined before and after an in-focus point.